What is the difference between the armor types?

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I've noticed that normally the armor bar is blue, but if you find a secret gold armor it will turn yellow. You can also still pick up a gold armor even if you're at 100 blue armor.

It reminds me of Doom and its dual-armor system, though unlike in Doom, it at least APPEARS that you can pick up blue armors to keep the gold, as long as it never hits zero.

But does anyone know what the exact difference is? I've seen armor act more like a second health bar even when it's blue in some cases, and of course in the heat of combat it's very difficult to ascertain how much anyone is hurting you. In Doom it was 25% protection and then 50% protection (Quake then added one more armor type that blocked a whopping 80%). I have no idea how it works here.